Factory Tach problem...Help please
Thread Starter
Instructor
Joined: Jun 2001
Posts: 116
Likes: 0
From: Ft. Worth TX
Factory Tach problem...Help please
Happy new year everyone. Ok I am having this problem with my factory tach that is driving me crazy. The problem is the tach is way off. At idle(900rpm) the tach displays 1800 rpm...and at 3000rpm the tach is pegged past 6000rpm. Its off all over the place. So my question is. What out of the distributor controls the Tach? Any help would be apprecitated.. Thanks
Drifting
Joined: Jul 2000
Posts: 1,861
Likes: 1
From: Bremerton WA
St. Jude Donor '09
Re: Factory Tach problem...Help please (NX1991)
Factory tachs are not reliable from a racing standpoint. The one on my 85 had a 400 rpm error. Not a big deal, right? Well, my 6300 RPM (read by factory tach) dyno pull actually went to 6700 rpm. Okay for a forged bottom end 383 but scary if I had cast parts. I'm putting in a real tach before I dyno the new engine.
Cory
Cory
Race Director
Joined: May 2001
Posts: 14,165
Likes: 9
Re: Factory Tach problem...Help please (CHarris85Vette)
Yes but your factory rev limiter should stop you at the right place, regardless of what your tach actually reads. I can understand not trusting the factory tach, but in the case of the original post something seems wrong. I know the tach is off, but it shouldn't be off that much.
Original Founding Member!
Joined: Jul 1999
Posts: 9,037
Likes: 88
From: Granby, MA
Cruise-In 1, 3, 9. 10 & 11 Veteran
Re: Factory Tach problem...Help please (NX1991)
hey NX.....guess what bud....you not alone my 1990 is identical!!!...and i been checkin into this prob for over 2 years now!!!
you peg the plastica against the mph gauge right?!!!...i do....over 6,000 and guess what....thats right buddy...scorp 508 will back me up on this....our rev limiters.....WE HAVE NONE.....the great L-98's rev limiter is at over 10,000rpms...and no iam not joking.....so you like me in the same boat!!
for me..only thing that i have not tried was changing out the whole cluster...which is what I have been told i have to do...
the rpm filters i have tried don't work...and the wires coming off the distributor..same deal...
so if you do find something out....PLEASE PLEASE let me know...send me an email.... xtremevette@hotmail.com....i will do the same for you....
you actually the first that i have talked to that has this bad an operating tach....like mine...
everybody else has only been off by like 500-600- which seems normal for a c4...
ahhh...its frustrating huh.....
hope we figure this out...lol
later
jeff
you peg the plastica against the mph gauge right?!!!...i do....over 6,000 and guess what....thats right buddy...scorp 508 will back me up on this....our rev limiters.....WE HAVE NONE.....the great L-98's rev limiter is at over 10,000rpms...and no iam not joking.....so you like me in the same boat!!
for me..only thing that i have not tried was changing out the whole cluster...which is what I have been told i have to do...
the rpm filters i have tried don't work...and the wires coming off the distributor..same deal...
so if you do find something out....PLEASE PLEASE let me know...send me an email.... xtremevette@hotmail.com....i will do the same for you....
you actually the first that i have talked to that has this bad an operating tach....like mine...
everybody else has only been off by like 500-600- which seems normal for a c4...
ahhh...its frustrating huh.....
hope we figure this out...lol
later
jeff
Burning Brakes
Joined: Nov 2000
Posts: 1,040
Likes: 2
From: BowlingGreen KY
Re: Factory Tach problem...Help please (XtremeVette)
I peg out the tach on my 90 very easy. You can send your whole cluster to Lingenfelter or Callaway, I don't remember which one it is but it's one of these guys, and they will install the 8000 rpm ZR-1 tach for $299. :cheers:
Race Director
Joined: May 2001
Posts: 14,165
Likes: 9
Re: Factory Tach problem...Help please (KYVette90)
You can send your whole cluster to Lingenfelter or Callaway, I don't remember which one it is but it's one of these guys, and they will install the 8000 rpm ZR-1 tach for $299. :cheers:
Melting Slicks
Joined: Apr 2000
Posts: 2,793
Likes: 46
From: Elizabeth City, NC
Re: Factory Tach problem...Help please (Nathan Plemons)
The tach in my Yellow 90 is the same way. At idle its at 1800 or so(according to Datamaster) and at 4000 its way over into the speedo. Its crazy. Not sure what to make of it either. Its gotta be the cluster from what I gather as well. Its just not worth the money to get another one. A shift light will do just fine. Good to hear about some others with the same problem. :smash:
Team Owner
Joined: Mar 2001
Posts: 54,124
Likes: 30
From: Cincinnati, Oh USA
Re: Factory Tach problem...Help please (NX1991)
My tach in my 87 is accurate. Those with problems need their cluster circuit boards repaired. There are a number of places around the country that repair them. I have been trying for a year now to get a schematic of the cluster boards with no success.
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From: DrunkDefender
CI 1-2-3-4-5-6-7-8 Veteran
CI-IV AutoX Champ
CI-IV Burnout Champ
St. Jude Donor '07-'10
Re: Factory Tach problem...Help please (NX1991)
It seems as though the 90'-91's have a much different problem than the tach just being off by a couple hundred rpm. I thought my car was sick or something since it ran so slow and felt very strong at 6000rpm. The dyno operator asked why I shut it down, I said "Because that was 6000rpm" nope, it was only about 3800-4000 rpm. Mine too is waaaay off. Its not a tach filter problem, thats an entirely different problem. From everyone I've taked to its because we have the lucky distinction of having the first cars with the new dash (new interior introduced in 1990) with its ANALOG tachometer. The problem is radio interference. We have the same gauges as the LT-1 so why dont they have this problem? They have the optispark, we have a good ole distributor which in this case lends itself more suseptible to electro-interference. Ijust put an aftermarket tach in with adjustable shiftlight and said to heck with it. Its a Sunpro tach, 3.5" face that fits nicely on top of the steering column in front of the original tach and still allows for full range movement of the tilt function.
